Chemical stability of cementitious matrices containing novel radioactive waste of 10B enriched boric acid – leaching behaviours of Ca, Si, Al, S, and Na

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Abstract
Boric acid enriched in 10B (EBA) is a novel neutron absorber that can be used in reduced quantities in nuclear power plants compared with boric acid with natural isotopic abundance (natural boric acid, NBA). Consequently, the amount of waste requiring solidification prior to disposal can also be reduced. However, our previous study demonstrated that, due to isotopic, elemental, and molecular differences of boron, the leachability of B—and thus the chemical stability—of cementitious waste forms produced from NBA and EBA liquid wastes differ significantly. In the present study, the leachability of the other major elements constituting the cementitious matrices, including Ca, Si, S, Al, and Na, was also investigated. Analytical techniques such as ICP-MS/OES, and XRD or SEM-EDX were applied to examine the effects of varying boric acid concentration and isotopic enrichment. The results showed that the leachability of non-structural elements (i.e., B and Na) is lower in EBA-containing specimens—by up to 29%—compared with NBA-based materials. In contrast, the leachability of the structural elements of the cementitious matrix (Ca, Si, S, and Al) increased, in some cases by up to 120%. These contrasting trends highlight the need for a comprehensive evaluation of the leaching behaviour of all major elements present in cementitious matrices when assessing the long-term durability of simulated final waste forms, rather than relying solely on boron leachability, as is commonly done.

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